In what is its most targeted search to date, the SETI Institute has scanned 86 potentially habitable solar systems for signs of radio signals. Needless to say, the search came up short (otherwise the headline of this article would have been dramatically different), but the initiative is finally offering some quantitative data about the rate at which we can expect to find radio-emitting intelligent life on Earth-like planets — a rate that's proving to be disturbingly low.

Indeed, by the end of its survey, SETI calculated that less than one-percent of all potentially habitable exoplanets are likely to host intelligent life. That means less than one in a million stars in the Milky Way currently host radio-emitting civilizations that we can detect.

A narrow-band search

The SETI researchers, a team that included Jill Tarter and scientists at the University of California, Berkeley, reached this conclusion after scanning 86 different stars using the Green Bank Telescope in West Virginia. These stars were chosen because earlier Kepler data indicated they host potentially habitable planets — Earth-like planets that sit inside their sun's habitable zone.

As for the radio bands searched, SETI looked for signals in the 1-2 GHz range, a band that's used here on Earth for such things as cell phones and television transmissions. SETI also constrained the search to radio emissions less than 5Hz of the spectrum; nothing in nature is known to produce such narrow band signals.

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Each of the 86 stars — the majority of which are more than 1,000 light-years away — were surveyed for five minutes. Because of the extreme distances involved, the only signals that could have been detected were ones that were intentionally aimed in our direction — which would be a deliberate effort by ETIs to signal their presence (what's referred to as Active SETI, or METI (Messages to ETIs)).

"No signals of extraterrestrial origin were found." noted the researchers in the study."[I]n the simplest terms this result indicates that fewer than 1% of transiting exoplanet systems are radio loud in narrow-band emission between 1-2 GHz."

Wanted: Alternative signatures

Despite the nul result, SETI remains hopeful for the future. Scanning potentially habitable solar systems is a fantastic idea, and it's likely the first of many such targeted searches. At the same time, however, SETI will have to expand upon its list of candidate signatures.

Indeed, the current strategy — that of looking for radio-emitting civilizations — is exceedingly limited. Even assuming we could detect signals from a radio-capable civilization within a radius of 1,000 light-years, the odds that it would be contemporaneous with us is mind-bogglingly low (the time it takes for radio signals to reach us notwithstanding).

And as we are discovering by virtue of our own technological development, the window of opportunity to detect a radio-transmitting civilization is quite short. Looking to the future, it's more than reasonable to suggest that alternative signatures — whether they be transmitted deliberately or not — be considered.

This is something SETI is very aware of, and the researchers said so much in their paper:

Ultimately, experiments such as the one described here seek to firmly determine the number of other intelligent, communicative civilizations outside of Earth. However, in placing limits on the presence of intelligent life in the galaxy, we must very carefully qualify our limits with respect to the limitations of our experiment. In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise. Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate. The search for extraterrestrial intelligence is still in its infancy, and there is much parameter space left to explore.

The paper is set to appear in the Astrophysical Journal and can be found here.